Which is nowhere in the book, it's only referred to as "ML language" so requires assumed domain knowledge such as (1)ML = Standard ML = SML/NJ (2)you know what a REPL is and that it has different commands on different platforms (Ctrl-Z to exit on Windows VS Ctrl-D in *nix) (3)you know commands to load your larger .sml programs into the REPL and all the pitfalls like needing to restart/empty the environment. In the course it's explained but not the book http://cs.wheaton.edu/~tvandrun/previous/spring19/cs243/ml.h...